A human rights group says security forces in western Myanmar massacred at least 40 Rohingya Muslims last week in a new outburst of violence.

Bangkok-based Fortify Rights says it's spoken to witnesses and other sources who confirmed the killings and said the dead included women and children.

Government officials have strongly denied the reports.

Rakhine state spokesman Win Myaing told Reuters that he's visited the area and found no evidence of mass killings.

Government-run media has also carried denials of a massacre.

If the killings are proven it would be the deadliest incident in western Rakhine state since October 2012, when ethnic Rakhine Buddhists fought minority Rohingya Muslims.

"The actual number of deaths may be higher but information is circumscribed by government-imposed restrictions on access to the area," Fortify Rights said in a statement.

Reports say the United Nations has also investigated the claims and says at least 40 people were killed.

Details of the latest attack began circulating after a January 13 clash between police and Rohingya villagers in Maungdaw township.

The area is remote and off limits to journalists, while access by humanitarian groups is strictly controlled.

State and national government spokesmen said police had been attacked.

But they denied reprisal killings.

UN humanitarian operations chief Valerie Amos says the government must take all necessary measures to protect civilians and allow humanitarian staff to reach the area.

"I also ask the government to immediately launch an impartial investigation into these events and to respect the rights of those arrested and detained in connection with this incident," she said in a statement.

If confirmed, the killings would take to at least 277 the number of people killed in religious conflict across Myanmar since clashes in June 2012.

More than 140,000 people have been displaced.

After emerging from 49 years of military rule in 2011, Myanmar has faced repeated sectarian violence that has marred its transition to democracy.